Link to categories on header main menu

Hi, I am new to WordPress.

I recently installed the free version of the white house template.

I realize that top level pages I create show up as links on the top header of the page. Can someone please help me change the links of those pages.

Basically, I want link on the header to be a blog category. I have 3 blog categories and I would like to link each category on the main header. Currently the links on the main header point to individual pages. The links on the sidebar point to the various categories I have setup.

I hope this makes sense.

Thanks in advance.